1. Field of the Invention
The present invention relates to an electromagnetic energy source or transmitter for borehole to surface electromagnetic surveying and mapping of subsurface formations.
2. Description of the Related Art
Electromagnetic methods to obtain data regarding subsurface earth formations and their constituent fluid contents have been used for several purposes. Among these have been petroleum reservoir characterization and front-tracking in enhanced oil recovery operations.
One of these electromagnetic methods has been what is known as the borehole-surface or borehole to surface electromagnetic method (BSEM). Two electrodes have been used in the borehole-surface electromagnetic energy method. The first electrode has been in a well borehole of what is known as the transmitter well, transmitting electromagnetic energy, and the other, which may be a ground electrode, has been at the earth's surface along with a receiver array. The receiver array has been located at spaced positions on the surface conforming to the reservoir of interest to detect the energy field after passage through the earth from the first or transmitter electrode.
In a typical operation, Borehole to Surface Electromagnetic (BSEM) utilized an electromagnetic source in the borehole and an array (typically 600-2000 or more) of receivers on the surface, thus allowing the mapping of the fluid (typically oil and water) distribution in large areas of the reservoir a few (2-4) kilometers away from the well in which the transmitter electrode had been positioned.
The transmitter electrode located in the well was activated at depths of interest. The signal emitted on activation could be a single frequency or multiple frequencies. The resultant electromagnetic field which then occurred was sensed in the time and frequency domains by the receiver array. Surveys of this type could then be repeated after passage of a period of time from the transmitter well to track the subsurface fluid migration.
An interface in a subsurface formation between solids and liquids produces induced polarization and frequency scattering responses to the emitted signals and the responses were received and recorded. The recorded data was processed and analyzed to map boundaries of subsurface reservoirs of interest and evaluate other nearby formations. The information obtained was important in assessing the sweep efficiency, or the percentage of original oil displaced from a formation by a flooding fluid, and in locating potential bypassed oil zones, thus ultimately increasing oil recovery.
So far as is known, no provision has been made to obtain a precisely accurate measurement of the depth position of the transmitter downhole. An indirect measurement was possible only from measurements of the length of cable passing from the cable reel or drum in the wireline truck into the well. However, this length measurement did not take into account elongation of the cable at increasing depths in the well. This gave rise to an inability to accurately determine well depth measurements of formations and correlate actual depth of the transmitter emissions of energy with data representative of subsurface conditions.
During BSEM surveying, other well logging operations with other well logging tools present in the well borehole were not, so far as is known, conducted. The purpose of this was so that the transmitter electrode could be easily moved to desired depths in the well. Thus, there was no capability to monitor downhole well conditions during the BSEM survey. Thus, so far as is known, no provision was made to detect incipient abnormal conditions which might provide advance notice of one or more of possible problems, such as overheating of the transmitter electrode, starting of an ignition in the well, a gas kick, overpressure, or the like.